Connect with us

variac/dc power supply repair

Discussion in 'Electronic Repair' started by Ryan Underwood, Mar 6, 2006.

  1. Hi,
    I just acquired a Lab-Volt model 189 bench supply, 1966 vintage. It has a
    passthrough AC output, a variac output, and a variable DC output.
    Unfortunately, the variable outputs do not work (0 voltage independent of
    knob setting). The pass through output does work though.

    The fuse is a screw-in type, black thumb screw reads "Fuse" and when it pulls
    out, there is a loose metal cylinder around the end of the fuse. I have no
    idea how to test this type of fuse or even what it is called.

    Anyone have pointers where to get started on this thing? The construction
    appears to be nothing more than a bunch of transformers and a knob. I'm not
    familiar with these variable type supplies.

    Thanks.
     
  2. Arfa Daily

    Arfa Daily Guest

    The fuse is easily enough tested using a simple ohm meter, or by
    substitution, but to be honest, if you do not have sufficient knowledge to
    be able to troubleshoot a simple power supply of this vintage, I really
    would not recommend that you start poking around inside, as there are
    dangerous voltages present in any mains power supply. Sorry to be so
    negative about this, but I'm only thinking of your safety ...

    Arfa
     
  3. Ross Herbert

    Ross Herbert Guest

    If it is similar to this model it shouldn't be too difficult to find
    out what is wrong with it. Looks pretty basic but I haven't actually
    seen one. BTW, if you can't determine whether a fuse is blown or not
    then you shouldn't be playing around inside it.
    http://it.stlawu.edu/~physics/labs/common/img/labvolt.jpg
     
  4. Agreed with the other postings, especially on the SAFETY issues.

    Most likely problme is a blown fuse (there might be nother one inside),
    next most likely would be a bad connection or short somewhere, next most likely
    might be a bad carbon brush on the Variac. There really isn't much
    else to go wrong, though a shorted rectifier might be the cause of the
    blown fuse.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  5. Guest

    Yeah, that's the same style of fuse. I just don't know how to test
    that type of fuse. I am only familiar with repairing PC power supplies
    and the glass/filament type fuses that they use.
     
  6. What do you mean by "that type of fuse". What type is it?

    In any case, you test that fuse like any other, with a multimeter on
    ohms or continuity. It should be read as close to 0 ohms.

    Respectfully, if you don't even know how to test a fuse, what are you
    going to use this power supply for, and do it safely?

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  7. Guest

    I am fully aware of how to test a fuse, given that I can find the
    terminals of the fuse, which is not usually an unreasonable assumption.
    If you look at the picture, it is a type of fuse that screws in. It
    is not like the "edison" type screw-in fuses. There is kind of a
    "rivet" in the end of it that, when removed, retains a metal cylinder
    that hangs loosely over the rivet. I can't figure out how the fuse is
    supposed to work, because when I've removed it, it physically becomes
    one big short because of the loose cylindrical part.

    I am fully aware of the hazards, but from the replies, you'd think I
    was prodding around in it with a kitchen knife while plugged in. All I
    am trying to do is test this fuse, and if disassembly is required, I
    have no plans to do that with it plugged into the wall or with an
    energized filter cap.
     
  8. It sounds to me like the fuse is missing.

    The fuse that goes in there is probably 1-1/4" long by 1/4" in diameter.
    When someone asks how to test a fuse, it is no wonder that people are
    questioning your experience.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  9. Ross Herbert

    Ross Herbert Guest

    It may simply be that the fuse holder is broken...

    If the fuse is a screw cap then you simply unscrew the cap
    anti-clockwise. The fuse will most likely be a 3AG (1-1/4" x 1/4"
    tubular glass). Usually the screw cap has a flat spring retainer to
    hold the new fuse when inserting it but these sometimes break or go
    missing butthis doesn't usually affect the function of the fuse
    holder. At the far end of the fuse holder the rear terminal is spring
    loaded so that pressure is applied to both ends of the fuse when the
    cap is screwed home. You can test to see if the spring exerts pressure
    by pushing a screwdriver blade down inside the fuse holder.

    I have found these fuseholders sometimes go open circuit usually
    through a terminal or phenolic body breakage internally. The only way
    to test the fuse holder is to disconnect (unsolder) one of the wires
    connecting to it (the far end one is easiest) and with a known good
    fuse inserted measure across both terminals of the fuse holder with
    your DMM. Naturally, you must ensure that no power is applied while
    doing any of this.
     
  10. James Sweet

    James Sweet Guest

    Are you sure there's even a fuse in there? Almost sounds to me like an
    empty holder.
     
  11. Guest

    Sam, save the flames for someone more deserving, please. I said that I
    have never seen THIS TYPE of fuse before, because I mistook what is
    apparently a broken fuse holder for the fuse itself. That is not
    equivalent to asking "how do I test a fuse?". As for "questioning my
    experience", go right ahead, I'm not going to get into a pissing match,
    but it is usually less effort to simply answer the question as posed.

    I openly admit that I rarely get to deal with nice test equipment. And
    I don't see where that means that I should simply accept that it is
    broken and not attempt to learn about it - just as I learned about
    microwaves, televisions, monitors, amplifiers or any other piece of
    electronics that could be dangerous if one does not employ common sense.
     
  12. Guest

    Yes, it appears to be an empty, broken fuse holder. When I first
    unscrewed it, I was confused and thought it was similar to the mains
    fuses that screw into the panel. A step in the right direction, thanks!
     
  13. James Sweet

    James Sweet Guest


    It doesn't even sound broken to me, often the fuse is all that holds the
    bottom contact down away from the cap, with no fuse at all installed all
    bets are off.
     
  14. Ross Herbert

    Ross Herbert Guest

    So, are you now sure that the problem was caused by a broken fuse
    holder in the first place?
     
  15. James Sweet

    James Sweet Guest


    Well there's no sense in arguing about it or belittling one another, I
    would however strongly recommend doing some research on the subject
    before mucking around with it too much. I do agree that a person should
    be able to clearly identify just about any type of fuse or fuse holder
    they come across before poking around in anything mains powered.
     
  16. Guest

    I got a service diagram from Lab-Volt after much consternation. Looks
    like the holder is supposed to be like that, when the holder screws in
    it both tightens up the "link" to the fuse as well as completes the
    connection. So the holder may not be broken, I'll just see if it blows
    another fuse before proceeding. Thanks for the helpful replies.
     
  17. Ross Herbert

    Ross Herbert Guest


    You are a worry... this is the first time you have even mentioned that
    the unit is "blowing fuses". Your previous posts inferred that the
    problem was with continuity related to this fuse, but this new
    information puts a whole new complexion on things. The very fact that
    the fuse is blowing infers there is nothing wrong with the fuse holder
    or the fuse. Assuming that you are using a properly rated delay fuse
    (slo-blo, or time delay fuse usually with a 'T' marking), and it is
    still 'blowing', then the fuse and holder are fine. The most likely
    problem is that there is some component following the fuse which is
    overloading it and causing it to do its intended job.

    Have you even opened the case to see if there is any obvious sign of a
    short circuit anywhere? Perhaps I shouldn't ask that if your attempt
    to repair the unit up till now is an indication of your knowledge.
     
  18. Smitty Two

    Smitty Two Guest

    The OP's claims to the contrary, it seems to me that he doesn't know a
    fuse from a fuse holder. Fine, ignorance is OK, and if he's a novice, I
    have no problem with that. But, his complete inability to see what he
    sees and communicate it clearly is exceptionally frustrating. One
    picture link would help us help him.
     
  19. It's of more concern that if he doesn't know a fuse from a fuse holder,
    he may not know the difference between a live wire and ground and may end
    up dead.

    --- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
    Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
    +Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
    | Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

    Important: Anything sent to the email address in the message header above is
    ignored unless my full name AND either lasers or electronics is included in the
    subject line. Or, you can contact me via the Feedback Form in the FAQs.
     
  20. Guest

    I know the difference between a fuse and a fuse holder in terms of
    their function. However, I have not seen every fuse holder nor every
    fuse. I jumped to a conclusion at the very beginning when I thought I
    was looking at some strange, obsolete type of *fuse* when I was looking
    at a *holder* of type which is apparently common and well known in test
    equipment. How should I know? That's why I asked. (You know, the
    holder says "Slo Blo" on it too.)

    To reply to the GP, with this knowledge of what is a fuse and what is a
    holder straightened out several posts ago, I concluded that it was
    given to me without a fuse. Isn't it a reasonable presumption then
    that it previously blew a fuse? What other reason would one have for
    removing a fuse? No, there are no obvious short circuits in that there
    are no loose wires. Since the presumption was that a fuse blew, that
    was the first thing to check. But I can't tell whether it will blow
    another fuse without putting another fuse in and powering it up, can I?

    I don't see what has been so difficult to understand about what I have
    posted. The largest part of the problem is that respondents would
    rather jump to conclusions about my mettle than ask for clarification
    if what I have posted is ambiguous or seems wrong. I try to anticipate
    replies when I post, but it is inescapable that descriptions will be
    lacking at times. Furthermore, I have visited this project
    approximately three times; since it is at home and I work all day my
    opportunities are limited.

    My experience is limited to consumer electronics component-swapping
    (including fuses). That's why I haven't seen this type of holder
    before - fuse holders in consumer equipment are almost invariably the
    flex metal type. I can take the appropriate precautions vs becoming a
    crispy critter when working on a monitor, a PC power supply, or a
    microwave, so why do I feel like I need a disclaimer at the end of
    every post I make about this power supply?

    I realize you get what you pay for in terms of soliciting advice, but I
    recall this group being a lot less hostile 5 years ago.
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-